This invention relates to an inverter for supplying power to an electroluminescent (EL) lamp and, in particular, to an inverter including low voltage transistors on a first semiconductor chip and one or more high voltage transistors on a second semiconductor chip for driving the EL lamp.
Many electronic watches include an EL lamp for backlighting the face of the watch, such as a liquid crystal display or an analog dial. An EL lamp provides glare-free, uniform lighting across the entire face of the watch, is relatively thin, and consumes very little power. The disadvantage of an EL lamp is the need for a high voltage alternating current to drive the lamp. An EL lamp typically requires eighty volts or more to produce the desired brightness and the battery in a watch typically has a voltage of three volts or less. An inverter is used to convert low voltage direct current from the battery into a high voltage alternating current for the EL lamp.
An electronic watch typically includes an oscillator, a counter or divider circuit, and other low voltage semiconductor devices. Also fitting within a watch case are a battery and a mechanical resonator for controlling the frequency of the oscillator. Adding an EL lamp to a watch not only adds the lamp itself but also adds several additional components that must fit within a case, such as an inductor and the inverter circuitry. Style considerations prevent a watch case from being arbitrarily enlarged.
Electronic watches of the prior art typically include a timer chip and an inverter chip. The timer chip performs the clock functions of the watch and the inverter chip powers the EL lamp. Custom integrated circuits are used to reduce size and cost and to minimize the number of parts. Size is further reduced by using bare chips; that is, chips that are not packaged.
A typical inverter circuit for watches includes what is known as a "flyback" or boost circuit in which the energy stored in an inductor is supplied to the EL lamp as a small pulse of current at high voltage. Specifically, an inductor and a switch transistor are connected in series across the battery in a watch. The junction of the switch transistor and the inductor is coupled to the EL lamp through a diode. When the transistor shuts off, the collapsing field in the inductor generates a high voltage pulse proportional to .sup..delta.i /.sub..delta.t. A series of such pulses charges the lamp and the lamp glows. The polarity of the high voltage pulses is reversed periodically by additional circuitry to produce alternating current. The switch transistor is turned on and off at high frequency, four kilohertz or more, to minimize the size of the inductor.
Although the basic inverter circuit is relatively simple, the actual implementation of the circuit is more complicated. Prototypes of an inverter are typically made with discrete devices or with standard integrated circuits but production quantities of an inverter are least expensive when as much of the inverter as possible is incorporated into a single semiconductor chip as a custom integrated circuit.
The process for manufacturing custom integrated circuit begins with "design rules" for laying out devices on a wafer. One design rule is the minimum line width that can be accommodated in a particular process. For example, high voltage devices typically have a minimum line width of 5 microns (5.0.mu. or 5.times.10.sup.-4 millimeters). A low voltage device, such as a logic array, may have a minimum line width as small as 0.3.mu..
A second aspect of designing a custom integrated circuit is the library of standard circuits that are used. These are small circuits whose fabrication and performance are well established and tested and the circuits are used as building blocks for producing a custom integrated circuit. For example, a flyback inverter implemented as a custom integrated circuit typically includes a plurality of transistors that provide ancillary functions such as timing, logic, coupling or buffering, pulse shaping, temperature compensation, and feedback for stability. The transistor count in a "real-life" custom inverter chip can exceed two hundred even though the basic circuit may be illustrated in a patent or other literature as containing only five or six transistors. The ancillary circuit elements significantly increase the size of the chip.
The majority of the transistors in an inverter operate at low voltage. An inverter also includes switching transistors that operate at high voltage and therein lies a problem. High voltage transistors are physically larger than low voltage transistors. High voltage transistors are not implemented with sub-micron design rules but with a minimum line width of about 5.mu., further increasing the size of the device. As used herein, a high voltage transistor is a transistor that can operate at ten volts or more.
Processes which produce fairly small low voltage transistors yield physically large high voltage transistors when all are made on the same chip. Conversely, processes which produce small high voltage transistors yield large low voltage transistors when all are made on the same chip. Either way, the area of the resulting chip is large.
As noted above, patents or other literature typically illustrate a basic circuit as it might be implemented in discrete form for proof of concept, not as implemented in a custom integrated circuit. Although the basic circuit is more easily understood because it is not obscured in a lot of needless detail, it is a misinterpretation of the literature to think that a commercially viable inverter requires so few components.
A commercially available, digital, electronic watch typically includes a clock chip having an area of about 25,000 square mils and an inverter chip having an area of about 3,700 square mils. A commercially available analog electronic watch typically includes a clock chip having an area of about 4,000 square mils. The cost of a chip is essentially a linear function of area: the more chips one can obtain from a single wafer, the lower the cost per chip. The electronic watch market is intensely price competitive and the cost of a watch "assembly", the inner workings without a case and wristband, is critical. Any reduction in cost becomes profit, either directly or as a sale obtained from a price reduction.
In the prior art, PCT application PCT/US93/04698 (McLaughlin et al.) disclose a microprocessor controlled pager including an inverter for an EL lamp. U.S. Pat. No. 5,093,612 (Herold) discloses a selective call receiver including a logic circuit and an inverter for powering an EL lamp. U.S. Pat. No. 4,529,322 (Ueda) discloses a booster circuit for powering an EL lamp. The booster circuit includes a CMOS controller and a bipolar driver circuit. U.S. Pat. No. 4,527,096 (Kindlmann) discloses driving an inverter from a counter/divider in a watch chip. The Kindlmann patent describes one of the few circuits known to be produced commercially. In its commercial form, the inverter includes many more transistors than shown in the patent drawings and includes high voltage transistors and low voltage transistors on a single chip. Unpatented, commercially available inverters for watches also include high voltage transistors and low voltage transistors on the same chip.
In view of the foregoing, it is therefore an object of the invention to reduce the area of the high voltage chip used in electronic watches having an EL lamp.
Another object of the invention is to provide an electronic watch including an inverter having low voltage control circuitry on a semiconductor chip, wherein the chip includes all the low voltage circuitry for the watch.